1. Field of the Invention
The present invention relates to a liquid crystal display and a method of manufacturing the same and, more particularly, to a liquid crystal display utilizing a condition in which liquid crystal molecules are vertically aligned when no voltage is applied and a method of manufacturing the same.
2. Description of the Related Art
Active matrix type liquid crystal displays (LCDs) that have been widely used include TN mode liquid crystal displays in which a liquid crystal material having positive dielectric constant anisotropy is aligned such that it is in parallel with surfaces of substrates and such that it is twisted at 90 deg. between the substrates which are opposed to each other. However, the TN mode has a problem in that it provides low viewing angle characteristics, and various studies are in progress in order to improve viewing angle characteristics.
As an alternative to this method, MVA (Multi-domain Vertical Alignment) method has been developed in which a liquid crystal material having negative dielectric constant anisotropy is vertically aligned and in which tilting directions of liquid crystal molecules are regulated by protrusions and slits provided on surfaces of substrates when a voltage is applied, and the method has successfully improved viewing angle characteristics.
An MVA type liquid crystal display will be described with reference to FIGS. 1A, 1B and 2. FIGS. 1A and 1B are perspective views showing the concept of the MVA type liquid crystal display. FIG. 2 is a conceptual diagram showing aligning directions of liquid crystal molecules on a pixel 7 of the MVA type liquid crystal display as viewed in a direction normal to a surface of a substrate thereof.
In the MVA type liquid crystal display, as shown in FIGS. 1A and 1B, a liquid crystal material (liquid crystal molecules) 5 having negative dielectric constant anisotropy is vertically aligned between two glass substrates 21 and 22. A pixel electrode connected to a TFT which is not shown is formed on the glass substrate 21, and an opposite electrode is formed on the other glass substrate 22. Protrusions 61 and 62 are alternately formed on the pixel electrode and the opposite electrode, respectively. Vertical alignment films which are not shown are formed on the pixel electrode and the opposite electrode which are not shown and on the protrusions 61 and 62.
When no voltage is applied to the liquid crystal molecules 5 with the TFT in an off-state, as shown in FIG. 1A, the liquid crystal molecules 5 are aligned in a direction perpendicular to substrate interfaces. When the TFT is turned on, an electric field acts on the liquid crystal molecules 5, and tilting directions of the liquid crystal molecules 5 are regulated by the structure in which the protrusions 61 and 62 are formed. As a result, the liquid crystal molecules 5 are aligned in a plurality of directions in one pixel as shown in FIG. 1B. For example, when the protrusions 61 and 62 are formed as shown in FIG. 2, the liquid crystal molecules 5 are aligned in each of directions A, B, C and D. Since the liquid crystal molecules 5 are aligned in a plurality of directions when the TFTs are turned on in the MVA type liquid crystal display, preferable viewing angle characteristics can be achieved.
In the above-described MVA method, vertical alignment films do not regulate the tilting directions of the liquid crystal molecules 5. Therefore, there is no need for an alignment processing step such as rubbing that is essential for horizontal aligning methods represented by the TN method. This is advantageous from the viewpoint of processing. in that the problem of static electricity and waste which occurs during a rubbing process is eliminated and in that no washing process is required after an aligning process. Further, since problems such as display irregularities attributable to variation of a pre-tilt will not occur, there is another advantage in that a cost reduction can be achieved through simplification of processes and improvement of yield.
Patent Document 1: JP-A-11-95221
Patent Document 2: JP-A-5-232465
Patent Document 3: JP-A-8-338993
Patent Document 4: JP-A-8-036186
Although the MVA method has various advantages as thus described, the simplification of processes, the improvement of yield and the cost reduction can be further encouraged if the existing step for forming vertical alignment films can be omitted. Further, although there is a need for forming vertical alignment films on mother glasses which are increasing in size as a result of the recent trend toward greater LCDs, a problem has arisen in that the existing alignment film printing apparatus may become unable to satisfy such a need.
In the existing methods for forming vertical alignment films, there is a phenomenon in which horizontally aligned domains called white lines can remain in a vertically aligned region. There is a need for reducing or eliminating such white lines to suppress reduction in contrast.